1. Field of the Invention
The present invention relates to a single-phase induction motor, and more particularly to a single-phase induction motor, which changes the number of turns of an auxiliary winding, through which a current flows, according to the operating mode of the motor, thereby increasing both the start torque performance when the motor starts and the efficiency of the motor when the motor runs in normal mode.
2. Description of the Related Art
A single-phase induction motor is a device that produces an alternating current on a winding installed on a stator, and produces an alternating magnetic field according to the current variation, thereby applying a torque to the rotor and thus obtaining a rotating force.
However, in the case where the motor receives single-phase AC power from a driving power source, the motor simply produces an alternating magnetic field in the axis direction of the winding, and thus it is not able to produce a rotating force. A start device is installed in the motor to start the motor. Typically, a capacitor-based start device is used as the start device.
FIG. 1 is a circuit diagram of a conventional single-phase induction motor. As shown in FIG. 1, the conventional single-phase induction motor is driven by a power source E, and includes a main winding M, an auxiliary winding S, a run capacitor Cr connected in series with the auxiliary winding S, and a subsidiary start device connected in parallel with the run capacitor Cr. The subsidiary start device is generally composed of a Positive Thermal coefficient element (hereinafter referred to as a “PTC”) 50. The PTC 50 is an element whose resistance varies according to temperature. The PTC 50 has a high resistance at high temperatures and has a low resistance at low temperatures.
The single-phase induction motor further includes a rotor 20, which is shown as a circle between the windings M and S in FIG. 2. A start capacitor (not shown) may be connected in series with the subsidiary start device.
The single-phase induction motor configured as described above operates in the following manner.
As the power source E supplies power to the motor, a current Im flows to the main winding M, so that the main winding M produces a main alternating magnetic field. Also, a current Is, whose phase is shifted by the run capacitor Cr, flows to the auxiliary winding S, so that the auxiliary winding S produces an auxiliary alternating magnetic field whose phase is different from the main alternating magnetic field produced by the main winding M. As the auxiliary winding S produces the auxiliary magnetic field with a different phase from the main magnetic field, a rotating magnetic field is produced, so that torque is applied to the rotor 20, thereby rotating the rotor 20.
In addition, since the PTC 50, which is used as a subsidiary start device, has a relatively low resistance when the motor starts, most of the current Is passing through the auxiliary winding S flows through the PTC 50 when the motor starts, thereby increasing the start efficiency of the motor.
On the other hand, when the motor runs in normal mode after a predetermined time from the start of the motor, the PTC 50 has a very high resistance, thereby opening a connection line, through which the PTC 50 is connected to the circuit, and thus preventing current from flowing to the PTC 50.
Accordingly, most of the current Is passing through the auxiliary winding Is flows through the run capacitor Cr, so that the rotor 20 rotates at synchronous speed due to the interaction between the rotor 20 and the magnetic fields produced by the main winding M, the auxiliary winding S and the run capacitor Cr.
FIG. 2 is a circuit diagram illustrating the connection state of wires of an auxiliary winding of a conventional single-phase induction motor. The conventional single-phase induction motor includes a stator 30 having a number of slots 1 to 14 and 1′ to 14′. A coil is wound on the stator 30 through the slots 1 to 14 and 1′ to 14′ to form a main winding and an auxiliary winding. The auxiliary winding includes two coils connected in parallel. The first coil (n3 to n7) of the auxiliary winding is formed by winding turns n3 to n7 on the stator 20 through slots 12, 3, 11, 4, . . . in the named order. The second coil (n3′ to n7′) is formed by winding turns n3′ to n7′ on the stator 20 through slots 12′, 3′, 11′, 4′ . . . in the named order. The two coils of the auxiliary winding are connected in parallel by connecting both ends of the first coil (n3 to n7) of the auxiliary winding respectively with both ends of the second coil (n3′ to n7′) thereof.
The run capacitor Cr is connected between the power source E and one-side ends of the two parallel coils of the auxiliary winding. The PTC 50 is connected in parallel with the run capacitor Cr. As a rotating magnetic field is produced by a current flowing through the coils, a rotating force is applied to the rotor 20, thereby rotating the rotor 20. In order to transfer the rotating force to the outside of the motor, a shaft 40 is placed in the rotor 20 at the center thereof so that it is oriented in the direction parallel to the axis of the rotor 20.
Both the operational and start efficiencies of the conventional single-phase induction motor have a significant influence on the performance of the motor. However, the operational and start efficiencies depend on the number of turns of the auxiliary winding. Specifically, if the number of turns of the auxiliary winding is large, the operational efficiency of the motor is increased but the start efficiency thereof is reduced. On the contrary, if the number of turns of the auxiliary winding is small, the start efficiency of the motor is increased but the operational efficiency thereof is reduced.
The conventional single-phase induction motor cannot achieve satisfactory start and operational efficiencies at the same time since the number of turns of the auxiliary winding is constant from when the motor starts running to when the motor stops running.